Optimize maps

Symptom – Noisy maps especially in areas that should have high, uniform composition of the element; results from the inherent statistical variation in the arrival rate of electrons on the detector

Solution – Acquire more electrons per pixel; typically you optimize a combination of beam current, exposure time and binning

Chromatic aberration

Symptom – Fringes along boundaries that separate dark and bright parts of the image; typically occurs when you do not use an objective aperture

Solution – Reduce the slit width and objective aperture to minimize the effects of chromatic aberration; however this will reduce the number of electrons collected and increase the shot noise

Image drift

Symptom – Unidirectional fringes along interfaces that separate dark and bright parts of the image and in areas with a lot of diffraction contrast

Solution – Repeat the drift correction function with different parameters or in manual mode; ensure sample is not damaging or shrinking; reduce exposure time and/or increase beam intensity

Image focus

Symptom – Blurred EFTEM maps at an energy loss

Solution – Focus the image

There are a number of general parameters you can choose to further optimize your EFTEM maps when you use Gatan Microscopy Suite® (GMS) software. These parameters affect all EFTEM acquisition modalities (e.g., SingleMap, MultiMap) and you can locate them in the EFTEM Mapping Preferences dialog.

Go to the EFTEM palette, then click the Setup button

The EFTEM Mapping Preferences dialog will then open

Drift measurement

falseDuring EFTEM data acquisition involving multiple images (e.g., summed image acquisition or mapping using multiple windows) the sample may drift, resulting in a spatial mismatch between images acquired successively. If not accounted for, this spatial mismatch will introduce artifacts into the final map or image. The EFTEM mapping routines have drift correction procedures that you can configure via the Drift Correction tab located in the EFTEM Mapping Preferences dialog.

Map computation

Automatically remove x-rays – Replaces erroneously high or low values in the source images with their local neighborhood average prior map computation

Hole-count threshold – Treats values below the set threshold as zero and omits them for the computation of maps; these typically are holes in the sample (e.g., regions with no sample)

Background model – Specifies the elemental mapping model you will use to compute the post-edge background contribution from the pre-edge images during creation of a 3-window elemental map

Options

falseWhen MultiMap is present, the Options tab is available. It contains the options which will globally affect all MultiMap acquisitions.

Colorize thickness map – When this option is selected the thickness map will be colorized

Automatically save and organize maps – Mapping routines will automatically save all acquired images at the end of acquisition using the Group Save options

Auto Exposure

falseThe Auto Exposure dialog contains a fast and robust auto-exposure and -binning routine to help simplify the acquisition of elemental maps with optimal image intensity. The behavior of the auto-exposure and -binning routines is dependent on both the preferences you specify as well as on the particular acquisition type.

Exposure – Sets the minimum and maximum exposure times the auto-exposure routine uses for EFTEM single and multi-mapping

Intensity – Determines the target intensity the auto-exposure routine will aim for, along with the minimum and maximum acceptable limits

Note: You can use Binning to trade signal-to-noise ratio (SNR) for number of pixels. Fewer pixels (higher binning) gives better SNR, but lower resolution. Typically 2x or 4x is used for edges up to 1 keV and 4x or 8x is used above 1 keV.

Energy offset method

The EFTEM modes adjust the TEM high voltage to offset the energy during acquisition. This helps ensure the lower column and the GIF remain in focus and aligned throughout the experiment. The other methods of offsetting the energy are used for spectroscopy and are described here.

false

Note: EFTEM at high energy losses is an incoherent imaging process. Conventional bright field TEM imaging is a coherent imaging process and lens defocus is used to add contrast to the image (for example Scherzer focus is typically used for HREM imaging). Incoherent images are only sharp for one value of focus (Gaussian focus). In addition, the large angular distribution of the energy loss electrons results in a very narrow depth of focus. This makes focusing at a reasonable large (~400 eV) energy loss prior to the start of acquisition critical for EFTEM mapping. Since the high voltage offset is used in EFTEM acquisition and the energy of the detected electrons stays fixed, once Gaussian focus is found, the focus does not need to be changed when the energy offset is changed.

Related content

Related FAQ's

When you see bright and/or dark bands at the interfaces of computed maps, this means your sample may be drifting:

Decrease exposure time (should not exceed 30 s for each frame)

If more than one frame is chosen select align images to remove the spatial drift

Ensure that spatial drift has been removed by aligning the images correctly before extracting elemental maps

Another cause is the application of excessive smoothing of the power law background exponent. This option is set in the EFTEM Mapping Preferences dialog. Consider reducing the smoothing half width or changing to a straight pixel-by-pixel power law model. The maps can be recalculated using the menu item “EFTEM | Compute elemental maps” without reacquiring the data.

Make sure the sample is thin by acquiring a relative thickness map. If it is thick (e.g., relative thickness >1), try moving to another sample area. Increase the number of collected electrons by one (or all) of the following:

If you see very sharp contrast that is not from the sample in bright areas, you may have saturated the detector in ones of the images. Decrease the detector exposure time and/or the sample illumination, then repeat.